[tune] fix pbt checkpoint_freq (#9517)

* Only delete old checkpoint if it is not the same as the new one

* Return early if old checkpoint value coincides with new checkpoint value

Co-authored-by: Kai Fricke <kai@anyscale.com>

python/ray/tune/checkpoint_manager.py

@@ -109,6 +109,10 @@ class CheckpointManager:
             return
 
         old_checkpoint = self.newest_persistent_checkpoint
+
+        if old_checkpoint.value == checkpoint.value:
+            return
+
         self.newest_persistent_checkpoint = checkpoint
 
         # Remove the old checkpoint if it isn't one of the best ones.